1. Field of the Invention
The present invention relates to a gas separating method far of separating valuable gas components such as nitrogen, oxygen, argon or the like from raw gas so as to extract these components by a cryogenic process and is also concerned with a gas separating apparatus suitable for the above-described gas separating method.
2. Description of the Prior Art
There is broadly known a conventional method wherein offgas separated by means of a gas separating apparatus is introduced into a heat exchanger in which the offgas is subjected to a heat exchange with raw air in order to cool down the raw air and at the same time the offgas is made to flow in an expansion turbine, thereby generating a cryogenic effect. Namely, a cryogenic process which utilizes the above-described low temperature offgas has been disclosed in the specification of Japanese Patent Laid-Open No. 79972 in 1980. The method disclosed therein is such that the low temperature offgas discharged from a nitrogen condenser of an air separating apparatus is fed into a heat exchanger wherein the offgas is thermally restored to an intermediate temperature and is then made to flow in the expansion turbine so as to expand in an adiabatic manner, thereby generating the cryogenic effect; and the low temperature offgas employed for generating the cryogenic effect is again led into the heat exchanger for the purpose of returning it to the normal temperature. With this method, the low temperature offgas discharged from the air separating apparatus is transferred intact to the expansion turbine via the heat exchanger. The pressure at an inlet of the expansion turbine is determined by the pressure of the low temperature offgas which is released from the air separating apparatus so that the former never exceeds the latter. For this reason, there exists a limitation on the extent to which the cryogenic effect can be generated per unit processing gas quantity.
Inasmuch as the extent of the cryogenic effect generated per unit processing gas quantity is small, it is necessary to employ a large amount of gas in order to generate a corresponding cryogenic effect which the gas separating apparatus requires. In the case of, for instance, a plant which extracts nitrogen and oxygen from air serving as raw gas by effecting a separating operation, this plant necessitates a step wherein a great amount of air is pressurized and is then supplied to the plant. Such being the case, it is necessary to prepare a large booster for pressurizing a large amount of raw air; and as a result, the consumption in energy (electricity in ordinary cases) whereby the booster is driven increases. The large sized equipment and the increased consumption in energy disadvantageously boost the costs in production of product gases such as nitrogen, oxygen, argon or the like which are produced (extracted) by means of the gas separating apparatus.